In an image scanning apparatus, an optical characteristic fluctuates because of aged deterioration of a light source, an environmental state of an optical system, and the like and an image scanning signal generated by a photoelectric conversion device (CCD) fluctuates. Therefore, in the past, the image scanning apparatus subjects the image scanning signal generated by the CCD to shading correction to always obtain the same image reproducibility even when the optical characteristic fluctuates.
For example, Japanese Patent No. 3255574 discloses an apparatus that changes the number of scanning lines of a shading correction plate according to copying magnification, averages scanned data into data for shading correction, and corrects fluctuation in an image scanning signal generated by a CCD using the data for shading correction.
However, when there is an area where dust adheres or air bubbles occur in the shading correction plate, if the area is scanned during shading correction, it is likely that correct data for shading correction cannot be obtained. If the data for shading correction is not correct, it is likely that correction unevenness occurs in a document scanning signal after correction and image reproducibility falls.
Therefore, there is a demand for development of a shading correction apparatus that can obtain, when there is an area where dust adheres or air bubbles occur in a shading correction plate, correct data for shading correction, prevent correction unevenness during shading correction, and improve image reproducibility even if replacement work or position adjustment work for the shading correction plate is not performed during manufacturing or during maintenance.